Exhaust gas recirculation technology is one of the main technologies for engines to achieve emission above national standard No. 4. Before recirculation entering an engine cylinder, exhaust gas shall pass through the EGR cooler to cool to a specific temperature. The working temperature of the EGR cooler at its heat side of high-temperature exhaust gas is usually 120° C.-30° C. and 80° C.-120° C. at its cool side of coolant. Therefore, during its working process, the wall temperature of the cooler pipes will be relatively high and the longitudinal thermal expansion will result in the deformation at the joint of cooling pipe and the main board, rendering the joint ineffective. The temperature at the exhaust gas inlet is usually 500° C.-730° C. Usually, the internal side of the inlet chamber directly contacts exhaust gas while the external side contacts surrounding air which results in that the temperature of the inside wall of the inlet chamber can reach 400° C.-600° C. The inlet side of the main board is under the direct impact of the high-temperature exhaust gas while the other side contacts the coolant. However, the highest temperature of the wall of the main board will reach 400° C.-600° C. as well. The deformation of the inlet chamber and main board caused by high temperature is another important reason for the ineffectiveness of the EGR cooler. Usually, the surrounding of the main board of the EGR cooler will be connected to the inlet chamber and water shell. When the inlet chamber and main board witness high-temperature horizontal thermal expansion, they will be constrained by the water shell with lower temperature which causes deformation. Severe deformation usually renders the EGR cooler ineffective. Therefore, reducing the wall temperature of the EGR coolers and releasing thermal expansion can be an effective solution to increase the reliability of the EGR coolers. Moreover, considering the design that the inner insulated pipe of the bellow of the gas inlet pipe is fixed to the inlet pipe only at one end, if the insulated pipe is relatively long, the vibration during operation will render the bellow or the insulated pipe ineffective.